Why Single-Pass Seawater desalination Recovery Is Typically Limited to 40–50%| Insights by AQUALITEK

Introduction

At first glance, designing a reverse osmosis (RO) desalination system with only 40–50% single-pass recovery may seem inefficient.

A common question from engineers and plant owners is:

Why not increase recovery to 60%, 70%, or even higher to reduce brine discharge?

In reality, single-pass recovery is one of the most carefully balanced design parameters in RO desalination.
Pushing it too high introduces serious hydraulic, chemical, and economic risks that outweigh the apparent water-saving benefits.

This article explains why 40–50% has become the industry standard for single-pass RO desalination.

1. Understanding Single-Pass Recovery in RO

What Is Single-Pass Recovery?

Single-pass recovery refers to:

•The percentage of feed water converted into permeate in one RO pass

Recovery=Permeate Flow/Feed Flow×100%

In seawater RO (SWRO):

•Typical single-pass recovery = 40–50%

•Overall plant recovery may be higher using multi-pass or staged designs

2. Rapid Salinity Increase Along the Membrane

Exponential Concentration Effect

As recovery increases:

•Salts rejected by the membrane remain in the concentrate

•TDS near the tail elements rises sharply

For example:

•At 50% recovery → concentrate ≈ 2× feed salinity

•At 65% recovery → concentrate ≈ 3× feed salinity

•At 75% recovery → concentrate > 4× feed salinity

This exponential effect drastically increases osmotic pressure and scaling risk.

3. Osmotic Pressure Limits Practical Recovery

Why Pressure Demand Rises Rapidly

Higher salinity → higher osmotic pressure → higher required operating pressure.

In seawater:

•Feed osmotic pressure ≈ 26–28 bar

•At high recovery, tail-end osmotic pressure can exceed 45 bar

Consequences:

•Required operating pressure may exceed membrane and pump design limits

•Energy consumption rises disproportionately

•Diminishing returns on water production

4. Scaling Risk Becomes Unmanageable

Supersaturation at High Recovery

Higher recovery causes:

•Calcium carbonate

•Calcium sulfate

•Barium/strontium sulfate

to exceed their solubility limits.

Even with antiscalant:

•Risk of irreversible scaling rises sharply above 50%

•Cleaning frequency increases

•Membrane life shortens

This is especially critical for:

•Warm seawater

•High alkalinity feed

•Limited pretreatment systems

5. Severe Concentration Polarization Effects

Local Conditions Are Worse Than Bulk Measurements

At higher recovery:

•Salt concentration at membrane surface exceeds bulk concentrate values

•Effective osmotic pressure is much higher than calculated

This leads to:

•Reduced permeate flux

•Increased fouling

•Localized scaling hotspots

6. Energy Efficiency Declines Beyond 50% Recovery

Nonlinear Energy Consumption

Increasing recovery beyond 50% results in:

•Rapid pressure increase

•Higher specific energy consumption (SEC)

Example trend:

•45% recovery → 3.0 kWh/m³

•55% recovery → 3.8 kWh/m³

•65% recovery → >5.0 kWh/m³

At some point, energy cost per cubic meter increases, defeating the purpose of higher recovery.

7. System Reliability and Operability Considerations

Higher single-pass recovery leads to:

•Narrower operating margins

•Increased sensitivity to feed water fluctuation

•Higher risk during transient events (temperature, turbidity spikes)

Designing at 40–50% provides:

•Stable long-term operation

•Predictable performance

•Tolerance to feed water variability

8. Why Not Use Multi-Pass Instead of Higher Single-Pass Recovery?

Industry-Preferred Strategy

Instead of pushing single-pass recovery:

•Use second-pass RO

•Treat concentrate separately

•Blend permeate strategically

This approach:

•Controls risk

•Optimizes energy use

•Extends membrane life

Conclusion

Single-pass RO recovery is not limited by membrane capability, but by system-level optimization.

The 40–50% design range represents a balance between:

•Energy efficiency

•Scaling control

•Osmotic pressure constraints

•Equipment protection

•Long-term reliability

Pushing recovery higher may look attractive on paper, but in real-world desalination plants, it almost always results in higher costs, higher risk, and shorter membrane life.